In any plastic optical fiber used for optical transmission, a core (inner layer) and a cladding (outer layer) each made of a clear resin are generally made into a form of concentric true circularities. In a plastic optical fiber having such a structure, light radiated from one of the ends of the fiber repeatedly undergoes total reflection at a phase boundary between the core and the cladding to be efficiently transmitted to the other end. Thus, the plastic optical fiber is effectively used as, for example, an endoscope for medical use, or an illuminating light-transmitting member for industry and automobiles. Any one of these members is used as a means to transmit light radiated from one end to the other end without leakage of light on the way. However, if this means can be caused to function as a linear radiator by causing light to leak from a middle (side surface) of the means in the longer direction thereof, the usage of the means can be developed for, for example, indoor and outdoor illumination, alternatives for neon signs or electrical displays and other decorations, and further to sensors.
As such a plastic optical fiber for side lighting, suggested is, for example, a side lighting optical fiber cable in which a light diffusion layer is formed as a layer outside a core and a cladding (for example, see Japanese Patent Laid-open Publication (JP-A) No. H05-341125 and JP-A No. H06-331830). However, formation of the light diffusion layer makes the side lighting optical fiber cable large in outer diameter, and further makes the cable high in elasticity. Consequently, there remain problems that the cable is low in bend nature, and is not easily handled or built for a specific usage.
In the meantime, as a plastic optical fiber for side lighting in which a means other than any light diffusion layer is used, for example, the following is suggested: a lighting plastic optical fiber in which the whole of a cladding or a desired region of a cladding from which light should be leaked is crystallized (see, for example, JP-A No. H06-118236). That technique applies a wet heat treatment to the whole of a cladding or a desired region of a cladding which should light, under wet heat conditions for crystallizing the whole or the desired region selectively. However, the cladding described in the specification thereof is poor in lighting performance.
A side light-leakage plastic optical fiber is also suggested in which a core has a noncircular cross-sectional form, and a cladding has a cross section in a truly circular form (see, for example, JP-A No. H09-258028). However, the optical fiber has problems that its lighting section is a partial region so that lighting therefrom is weak and further the fiber has, in accordance with a direction along which the optical fiber is viewed, a region which does not light. Furthermore, suggested is, for example, an optical fiber having an exposure region formed such that a cladding is removed to make a core exposed (see, for example, JP-A No. 2006-039287). However, that optical fiber has problems that the exposure region makes the plastic optical fiber low in mechanical strength and gives uneven lighting.
Thus, it could be helpful to provide a lighting plastic optical fiber from which light is emitted evenly in the longer direction of the fiber.